1. Field of the Invention
The present invention relates to a laser surface treatment using an ultra-intense short-pulse laser beam, and particularly for forming a fine periodic structure, which is effective to reduce reflection loss on the surface of an optical material.
2. Disclosure of the Prior Art
As a technique of forming a fine periodic structure on a surface of transparent substrate by use of a laser beam, for example, Japanese Patent Early Publication [KOKAI] 2003-57422 discloses a method of forming a periodic microstructure by irradiation of femtosecond-pulse laser beams. This method is essential to use two femtosecond-pulse laser beams interfered with each other, which are irradiated to a substrate to form a hologram diffraction grating on the substrate. The obtained diffraction grating has a periodic structure with a minimum average size of 5 to 200 nm. In this case, however, there is an inconvenience that the positional adjustment of an optical system must be performed with high accuracy such that the two femtosecond-pulse laser beams are interfered to each other. In addition, there is a problem that a large area treatment is difficult because of using the interference phenomenon.
By the way, when light propagating in an optical medium having a refraction index is incident on another optical medium having a different refraction index, reflection loss is generally caused at the interface between these optical mediums. The reflection loss increases as a difference of refraction index between the optical mediums becomes larger. In addition, under the condition that the light is incident on the optical medium having a small refraction index from another optical medium having a large refraction index, when the incident angle exceeds a threshold value (total reflection angle), total reflection occurs, so that the light can not propagate into the optical medium having the small refraction index. For example, under the condition that the light propagates from sapphire having the refraction index of 7.7 to the air having the refraction index of 1, when the incident angle is at right angle, the reflection loss is 7.7%. On the other hand, when the incident angle exceeds 34.4 degrees, the total reflection occurs. Thus, it is a subject to be solved to reduce the reflection loss in optical members using the optical transparency.